Hitherto, electrophotographic photoreceptors being photosensitive to visible rays have been widely used in copying machines, printers, and the like.
For such electrophotographic photoreceptors, inorganic photoreceptors having a photosensitive layer composed principally of an inorganic photoconductive material, such as selenium, zinc oxide, or cadmium sulfide, are widely used. However, these inorganic photoreceptors are not always satisfactory in various performance characteristics, such as photosensitivity, thermal stability, humidity resistance, and service durability, which are required of an electrophotographic photoreceptor for use in copying machines and the like. for example, selenium is liable to become crystallized under heat or by stains with fingerprints or otherwise, thus possibly deteriorating the foregoing characteristics of the electrophotographic photoreceptor. Photoreceptors in which cadmium sulfide is used are found unsatisfactory in humidity resistance and durability, and those in which zinc oxide is used have a disadvantage in respect to durability. Further, electrophotographic photoreceptors in which selenium or cadmium sulfide is used incur considerable limitations in manufacturing and in handling.
To overcome these problems on such inorganic photoconductive materials, attempts have been made to use various kinds of photoconductive materials in a photosensitive layer of an electrophotographic photoreceptor and, in recent years, various research and development works have been actively made which are directed to this end. For example, Japanese Patent Examined Publication No. 10496/1975 discloses an organic photoreceptor having a photosensitive layer containing poly-N-vinylcarbazole and 2,4,7-trinitro-9-fluorenone. However, this photoreceptor is still unsatisfactory in respect of sensitivity and durability. In view of this deficiency there has been developed an electrophotographic photoreceptor of the function divided type in which two separate layers are provided in place of a photosensitive layer, one as a carrier generating layer and the other as a carrier transporting layer, the two layers respectively containing a carrier generating material and a carrier transporting material. According to this arrangement, a carrier generating function and a carrier transporting function can be separately allotted to different materials so that the materials assigned to the different functions can be individually selected from wide ranges of materials. Therefore, electrophotographic photoreceptors having any desired performance characteristics can be comparatively easily produced. Thus, it is expected that an organic photoreceptor can be provided which has high sensitivity and high durability.
A number of different materials have hitherto been proposed as carrier generating materials effective for use in a carrier generating layer of such function divided type electrophotographic photoreceptor. The use of an inorganic material as such is disclosed in, for example, Japanese Patent Examined Publication No. 16198/1968 in which amorphous selenium is used. A carrier generating layer having an amorphous selenium content is used in combination with a carrier transporting layer containing an organic carrier transporting material. However, the carrier generating layer which comprises amorphous selenium involves the difficulty that, as already mentioned, the selenium is liable to become crystallized under heat or otherwise with the result of the layer being deteriorated in its performance characteristics. For use of organic materials as carrier generating materials, organic dyes and organic pigments may be mentioned by way of example. Photoreceptors having a photosensitive layer containing, for example, a bisazo compound are already known through various publications including Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) Nos. 37543/1972, 22834/1980, 79632/1979, and 116040/1981.
However, such known bisazo compound is of low sensitivity to light rays in a long wavelength range, though it exhibits comparatively good sensitivity in a short or medium wavelength range. Therefore, it has been difficult to use such compound in laser printers employing a semiconductor laser beam source on which expectations for high reliability are placed.
Gallium-aluminum-arsenic (Ga-Al-As)-based light emitting elements which are widely used today as a semiconductor laser have an oscillation wavelength of more than 750 nm or so. Various attempts have hitherto been made to provide an electrophotographic photoreceptor which is highly sensitive to light beams of such a long wavelength. For example, one approach considered is that a sensitizer is added to a photosensitive material, such as Se or CdS, which is highly sensitive to rays in a visible light range, to thereby make the material sensitive to long wavelength light rays. With this approach, however, there is some problem yet to be solved, because Se and CdS are, as earlier mentioned, not well resistant to such environmental conditions as temperature and humidity. Whilst, many known organic photoconductive materials have a limitation in sensitivity in that, as already stated, they are usually only sensitive to visible light rays within a wavelength range of not more than 700 nm, and there are few such materials which have good sensitivity to light rays in a longer wavelength range.
Of these materials, phthalocyanine compounds as one type of organic photoconductive materials are known as having a wider sensitivity range, than other organic photoconductive materials, which extends into a longer wavelength range. Phthalocyanines of various crystal forms have been found in the process of .alpha.-type phthalocyanines being converted into .beta.-type phthalocyanines having a stable crystal form. As an example of such phthalocyanine compound having good photoconductivity, mention may be made of .tau.-type metal-free phthalocyanines as disclosed in Japanese Patent O.P.I. Publication No. 182639/1983. In an X-ray diffraction spectrum as given in FIG. 10, such -type metal-free phthalocyanine shows Bragg angle 2 peaks at 7.6, 9.2, 16.8, 17.4, 20.4, and 20.9 degrees respectively in relation to a CuK.alpha. characteristic X-ray (wavelength 1.541 .ANG.) (which X-ray is hereinafter referred to as CuK.alpha. (1.541 .ANG.)).
In an infrared absorption spectrum, there are four intense absorption bands in a wavelength range of 700 to 760 cm.sup.-1 in which an intensemost band is seen at 752.+-.2 cm.sup.-1, two absorption bands of substantially same intensity in a wavelength range of 1320 to 1340 cm.sup.-1, and a characteristic absorption band at 3288.+-.2 cm.sup.-1.
However, the difficulty with such .tau.-type metal-free phthalocyanine is that production of same requires a complicated and difficult process such that an .tau.-type phthalocyanine is subjected to wet kneading together with grinding auxiliaries, such as sodium chloride and the like, and an inert organic solvent, such as ethylene glycol, at 50.degree.-180.degree. C., preferably at 60.degree.-130.degree. C., for 5 to 20 hours. Therefore, it is not always possible to obtain a .alpha.-type phthalocyanine having a given crystal form; accordingly, where such phthalocyanine is used as a carrier generating material, the resulting electrophotographic photoreceptor has no sufficient characteristic stability.
There are also known other phthalocyanine compounds, for example, .chi.-type metal-free phthalocyanines as disclosed in Japanese Patent Examined Publication No. 4338/1974. As compared with aforesaid .chi.-type metal-free phthalocyanines, the .chi.-type metal-free phthalocyanines are easier to produce and have greater crystalline stability and higher potential stability to repeated use in the case where they are used as carrier generating materials in electrophotographic photoreceptors, but still they are found unsatisfactory.
Now, known photoreceptors in which an organic photoconductive material is used are usually employed in negative charging application.
The reason for this is that where such a photoreceptor is used for negative charging, greater hole mobility is available in a carrier, and this fact is advantageous from the standpoint of photosensitivity or the like.
However, it has been found that such use for negative charging involves the following problems. One problem is that ozone is likely to be produced in the atmosphere during the process of negative charging by a charging unit, which leads to deterioration on the ambient conditions. Another problem is that the development of a photoreceptor for negative charging requires a positive polarity toner which is difficult to produce in view of its triboelectric sequence relative to ferromagnetic carrier particles.
As such, it has been proposed to use for positive charging a photoreceptor in which an organic photoconductive material is used. For example, there has been suggested a positive-charging type photoreceptor including a carrier generating layer and a carrier transporting layer placed thereon which is formed on a material having high electron transporting capability. The trouble with this photoreceptor is that the carrier transporting layer contains trinitrofluorenone or the like, such a material is carcinogenic. Whilst, it may be conceivable that a positive-charging type photoreceptor comprises a carrier transporting layer having high hole transport capability and a carrier generating layer placed thereon. Such layer arrangement is not suitable for practical purposes because a very thin carrier generating layer is present on the top side, which means lack in abrasion resistance.
In U.S. Pat. No. 3,615,411 there is disclosed a positive-charging type photoreceptor in which thiapyrilium salt (a carrier generating material) is used in such a way that it can form a eutectic complex in conjunction with polycarbonate (a binder resin). However, this photoreceptor has a drawback that it is liable to excessive memory effect and ghost image formation. The specification of U.S. Pat. No. 8,357,989 discloses a photoreceptor containing phthalocyanine. This photoreceptor has difficulties in that phthalocyanine is liable to characteristic changes depending upon its crystal form and must be strictly controlled with respect to its crystal form, and in that phthalocyanine does not provide any good sensitivity to light rays of short wavelength and is a cause of excessive memory phenomena. Therefore, it is unsuitable for use in copying machines in which light rays in a visible light wavelength range are used as a light source.
For aforesaid reasons it has been considered to be unrealistic to employ for positive charging any photoreceptor in which an organic photoconductive material is used, and hence such photoreceptor has been employed exclusively for negative charging.
The primary object of the present invention is to provide a photoreceptor which has sufficient sensitivity to light rays in a comparatively long wavelength range, such as semiconductor laser beams, and which is operable under positive charging and, in particular, less liable to ozone generation and superior in various performance qualities, such as abrasion resistance, potential stability, and memory and residual potential characteristics.